1. Field of the Invention
The present invention relates to an adhesive sheet for laser dicing used for supporting and fixing a workpiece when forming a reformed region inside the workpiece by light absorption ablation of laser beam and separating into individual pieces. The invention also relates to a manufacturing method of small pieces of element by separating into individual chips by light absorption ablation of laser beam by using various workpieces, such as circuit board, semiconductor wafer, glass substrate, ceramic substrate, metal substrate, light emitting or detecting element board of semiconductor laser, MEMS substrate, and semiconductor package.
2. Description of the Related Art
In manufacturing process of semiconductor related products, various circuits are formed on a substrate, and the surface is treated, and then the semiconductor substrate and the like is cut and separated (diced) to manufacture small pieces of element (for example, semiconductor devices, etc.). Specifically, an adhesive sheet is glued to the substrate, the substrate is diced by using a blade, and the adhesive sheet is expanded to separate into small pieces of element.
Recently, causing a little heat damage, the dicing method of semiconductor substrate by light absorption ablation by laser beam capable of processing at high precision is noticed as precise cutting method.
Technically, for example, a method is proposed for supporting and fixing a workpiece on a dicing sheet, and dicing the workpiece by laser beam (Japanese unexamined patent publication No. 2002-343747). The dicing sheet is composed of a base material including a support sheet, and an adhesive layer disposed at one side surface of the base material, and the adhesive layer can be cut off by laser beam, and the support sheet cannot be cut by laser beam.
Other method is proposed for dicing a semiconductor wafer by combining water microjet and laser (Japanese unexamined patent publication No. 2003-34780). On one side of a base material, a non-radiation curing type adhesive layer and a radiation curing type adhesive layer are disposed, and the base material allows to pass jet water stream of water jet, and the non-radiation curing type adhesive layer is interposed between the base material and the radiation curing type adhesive layer, according to this adhesive tape for laser dicing.
A further proposal is a laser processing method comprising a step of fixing a workpiece to be processed on a sheet of an adhesive surface, and a step of emitting laser beam by focusing on a spot inside the workpiece to be processed, and forming a reformed region inside the workpiece to be processed along an intended cutting line of the workpiece (Japanese unexamined patent publication No. 2003-33887).
When emitting laser beam, in order to form a reformed region at a uniform height inside the workpiece (the workpiece to be processed), it is required to focus the laser beam precisely in the intended area. In dicing by using an ordinary blade, thickness fluctuations of the workpiece of about several microns do not matter, but in the case of laser dicing, thickness fluctuations of the workpiece of about several microns may cause to deviate the focusing region of laser beam, and height of reformed region is not uniform, and height fluctuations in the reformed region may make it difficult to separate the workpiece into individual chips.
A certain laser dicing apparatus is equipped with a height adjusting mechanism, and the problem as described above may be solved by using such apparatus, but it takes much time in fine adjustment of height, and the production efficiency is poor, and it is not practicable. A workpiece having a large curvature may be machined somewhat by the laser dicing apparatus, but a workpiece having small undulations cannot be machined by the laser dicing apparatus.
Further, when separating into chips by expanding, depending on the adhesive sheet to be used, only the peripheral area of the sheet is expanded and the central area of the sheet is not expanded sufficiently, the central chips may not be separated neatly into individual pieces.